Cellulosic asymmetric "skinned" fluid separation membranes are known. See Lee U.S. Pat. No. 4,527,999. It is also known that the flux or fluid rate of flow through such membranes is inversely proportional to the thickness of the skin or thin, finely porous surface layer thereof. For this reason, it is highly desireable to fabricate such asymmetric membranes with as thin a skin layer as possible. In the above-mentioned U.S. Pat. No. 4,527,999 there is disclosed a method of casting a cellulose acetate asymmetric membrane and post-treating the same with a nonsolvent such as heptane, the heptane containing in solution a minor amount of a swelling agent such as water, followed by drying the treated membrane while constraining the side opposite the skinned side, the constraint taking place by the membrane adhering to a glass substrate. However, the so-prepared membranes still suffer from very poor selectivity.
The present invention provides a simple and efficient method of obtaining high flux, highly selective asymmetric cellulosic fluid separation membranes.